multiple servers and one domain

Hi, I'm planning my first installation of Zimbra and I'd like to understeand how to manage this situation:
I have one domani (example.tld) and 30 users. I have to configure two Simbra servers. 10 users mailboxes have to be stored on one server and 20 mailboxes on the other one. The users can send emails to all the others users but is a user is stored in the same server the mail won't to be routed to the other server.
I'm reading the manuals and seems to be that the only solution would be using the load balancing. is it right?

If a user know where is stored his account is it possible to use an environment without load balancer (by typing the right server name)?